In the past, various different types of past or prior art control devices, such as fluid pressure actuated switches for instance, were utilized in an automotive type air conditioning system to control the energization and deenergization of a clutch actuated compressor in such system in response to a preselected low and high value of fluid pressure measured at a preselected point in such system, such as in an accumulator of such system for instance.
Some of the past control devices were provided with a housing having an opening therethrough, and a movable fluid pressure responsive member, such as a diaphragm or the like for instance, was sealably interposed across the opening in the control device housing so as to define therein a pressure fluid chamber and a switch means accommodating chamber adjacent the opposite sides of the diaphragm, respectively. A round or disc-shaped snap disc movable between a stable configuration and an unstable configuration thereof was provided with a circumferential edge seated in the switch means accommodating chamber on the control device housing generally about the opening therein and adjacent the diaphragm, and a switch means operable generally between make and break positions for controlling a circuit through the control device was disposed in the switch means accommodating chamber generally adjacent the snap disc. Thus, the diaphragm was movable in response to a preselected fluid pressure acting thereon in the pressure fluid chamber to effect the application of a motive or applied force to the snap disc thereby to cause a snap-action movement of the snap disc from a stable configuration thereof toward a generally inverted or unstable configuration thereof. When the snap disc was so moved to its unstable configuration it effected the movement of the switch means from its break position to its make position. Thereafter, in response to a reduction of the fluid pressure in the pressure fluid chamber and a corresponding reduction in the motive force acting on the snap disc, the snap disc returned with snap-action from its unstable configuration to its stable configuration thereby to permit return movement of the switch means from its make position to its break position. Of course, in its make and break positions, the switch means completed and interrupted the circuit through the control device which effected the energization and deenergization of the aforementioned clutch controlled or actuated compressor, respectively.
In these past control devices, the round snap disc was formed from a generally thin strip or sheet of metallic material which may have had a directed grain orientation, such as for instance a stainless steel. In its assembly position, it is believed that the round snap disc was randomly arranged without regard to the grain orientation direction thereof so that the outer circumferential edge of the snap disc was disposed on a generally round or annular seating surface on the control device housing, and such seating surface was split or interrupted generally at diametrally opposite sections thereof. Also, a pair of oppositely spaced apart or split abutments were provided on the control device housing respectively adjacent the split seating surfaces and the round snap disc for abutting engagement therewith in order to positively limit the applied force movement or stroke of the round snap disc from the stable configuration to the unstable configuration thereof, as previously discussed. Thus, the round snap-disc was unsupported generally at the interrupted sections of the split-seat and also generally across the split abutments when engaged therewith.
During calibration testing of a plurality of the aforementioned prior art control devices in which the round snap disc was believed to be randomly seated in its assembly position therein without regard to the grain orientation direction of the snap disc, rather erratic calibration changes of the snap disc in most of such tested prior art control devices were encountered with most of such calibration changes exceeding a set limit or preselected allowable calibration change of the calibration test. Thus, it is believed that at least one of the disadvantageous or undesirable features of the prior art control devices was that the aforementioned unsupported engagement or arrangement of the snap disc on the split seat and/or the split abutments therefor caused the snap disc to assume a permanent set or deformation or the like. Another disadvantageous or undesirable feature of the prior art control devices which is analogous to the aforementioned disadvantageous feature is believed to be that the aforementioned deformation of the snap disc upon its engagement with the stops therefore deleteriously or adversely affected the calibration of the snap disc.
The calibration test to which the prior art control devices were subjected first involved ascertaining the initial "on" and "off" fluid pressures. The initial "on" fluid pressure is that necessary to effect the snap-action movement of the snap disc from the stable configuration to the unstable configuration thereof, and the initial "off" fluid pressure is that reduced fluid pressure at which the snap disc returns or snaps from the unstable configuration to the stable configuration thereof. Upon ascertaining these initial "on" and "off" fluid pressures at which the snap disc is actuated, a test fluid pressure of generally about 200 psig was applied to the past control devices to effect the snap-action movement of the snap disc to its unstable configuration in abutting engagement with the stops therefor, and while so pressurized, the past control device was subjected or heated to a test temperature of approximately 250.degree. F. for a test period of about two hours. At the end of this test period, the past control device was permitted to cool to about room temperature, and thereafter, final "on" and "off" fluid pressures of the past control device were ascertained in the same manner as discussed above with respect to the initial "on" and "off" fluid pressures. Change in the calibration of the snap disc in the past control device was ascertained by comparing the differential between the respective initial and final "on" and "off" fluid pressures, and of course, if either of these differentials was greater than a preselected test calibration change (1.5 psi), then the past control device failed the test and was unacceptable for use in the aforementioned automotive air conditioning system.